Preparation of thorium intermetallic compound dispersion



No Drawing. Filed Aug. 17, 1960, Ser. No. 50,095 4 Claims. c1. 75-12217The invention relates to a method of preparing a dispersion of thoriumbismuthide or thorium-lead compound in bismuth lead or bismuth-leadalloy and more particularly relates to a method of preparing adispersion of such a thorium intermetallic compound in fine particulateform.

In nuclear reactors neutrons escaping from the core are often utilizedto breed, or form, additional fuel material. Breeding consists mainly ofexposing a blanket material such as thorium to the escaping neutrons.Very fine dispersions of thorium bismuthide in bismuth or bismuth-leadalloy or thorium-lead compound in lead or lead-bismuth alloy are anadvantageous form of thorium which is useful as a fluid blanketmaterial.

A desirable property of a thorium intermetallic compound dispersion tobe used as a fluid blanket is that it be pumpable. Heretoforedifficulties have been encountered in avoiding dense compact crystallineforms, or grossly enlarged platelets which are up to centimeters indiameter. Neither of these forms are readily pumped.

It is therefore a principal object of the invention to provide a methodof preparing a readily pumpable dispersion of thorium intermetalliccompound in fine particulate form in bismuth, lead or bismuth-leadalloy.

The invention is predicated on the discovery that upon heating an alloyconsisting of about 10 to 50 percent by Weight of thorium the balance amagnesium metal, thereby to sublime or distill away the magnesium metaland leave a thorium sponge, and thereafter on heating the so-preparedthorium sponge in contact with a low melting metal selected from thegroup consisting of bismuth, lead or bismuth-lead alloy, a fineparticulate, pumpable dispersion of thorium intermetallic compound isobtained.

For the purposes of the specification and claims the term thoriumintermetallic compound refers to the intel-metallic compounds of thoriumwith bismuth and with lead.

In carrying out the method of the invention an alloy of thorium andmagnesium is provided by heating together under an inert atmosphere therequisite amounts of the metals to form an alloy consisting of about 10to 50 percent by Weight of thorium; the balance being magnesium metal.

For the purpose of the specification and claims a magnesium metal isdefined as a metal selected from magnesium and magnesium alloyed with upto 5 Weight percent of a volatile metal selected from the groupconsisting of calcium, sodium, zinc and mixtures thereof.

In accordance with the invention, an alloy of thorium and magnesiummetal may also be prepared from a compound of thorium, preferablythorium fluoride. The thorium compound is heated together with an excessof molten magnesium metal under an inert atmosphere to effect thereduction of the thorium compound, the soformed thorium alloyingdirectly with the excess of molten magnesium metal. Intimate contact andreactivity of the relatively high melting thorium compounds with therelatively low melting magnesium metal are generally facilitated bytaking up the thorium compound in a saline composition which is thenadmixed with molten magnesium metal. As an example, according to U.S.

Patent No. 2,678,267, thorium fluoride is admixed with NaCl and KCl toform a saline mixture which is melted and reacted with molten magnesiumin the preparation of magnesium-thorium alloy.

In any event, the thorium alloy selected or provided by preparationthereof should contain from about 50 to percent by weight of magnesiummetal, more preferably 70 to 85 percent in order to form the desiredporous sponge-like thorium mass during the sublimation'step. In orderthat a thorium dispersion may be formed therefrom in reasonably shorttimes, such as 1 or 2 hours, it is essential that the residual thoriumobtained in the present process have a particulate, skeletal, orcellular form in which the average shortest dimension of particle orcell wall is relatively small, e.g., less than 0.1 inch. Preferably thethorium has an average shortest dimension in the range of about 0.02 to0.05 inch.

Sublimation or distillation of the volatile light metal is carried out,broadly speaking, in a manner generally well understood in the art ofpurifying volatile metals. The thorium alloy is placed in a suitablecontainer, such as a molybdenum boat, which is placed in a sublimer,i.e., a horizontal furnace adapted to be evacuated and having anarrangement to collect the metal subliming or distilling from thethorium alloy. The furnace is generally evacuated to a reduced pressurelower than about 0.1 mm. of Hg and heating is begun to selectivelyvolatilize the said magnesium metal from the thorium.

Melting of the thorium once it is substantially magnesium-free is to beavoided because the formation of a porous residue is thereby prevented,resulting in either a dense sponge or massive thorium, neither of whichis readily dispersed in bismuth or lead. However, at the beginning ofthe heating period the melting of low-melting magnesium-thoriumintermetallic compounds is not deleterious and magnesium distills fromthe molten alloy. If it is desired tocarry out the preparation entirelyby the sublimation process, heating is commenced at a temperature atleast slightly below the melting temperature of the thorium alloy. Inany event, the temperature is gradually increased over a period of hoursto a temperature in the range fo 650 to 900 C. Temperatures above 800 C.are employed to assure more complete removal of the magnesium metal aswell as to cause sintering of the residual thorium into a fairlycoherent mass. If the thorium residue is not heated to a high enoughtemperature, it is crumbly and unsintered and tends to be pyrophoric onexposure to air. If the thorium intermetallic compound dispersion is tobe prepared in the same furnace in the absence of air, the stability inair is not important and a final temperature of about 750 to 800 C. issatisfactory. At 700' to 800 C. the sublimation process is usuallycompleted in 5 to 10 hours, depending upon the amount of alloy. Largeramounts of alloy require longer heating periods.

The resulting sponge is allowed to cool and is then dispersed inbismuth, lead or any alloy of bismuth and lead, any of which may beemployed as a blanket material solvent or dispersant medium in a nuclearreactor system.

If lead is employed, a dispersion of a thorium-lead intermetalliccompound, such as ThPbg, is obtained. In bismuth and most bismuth-leadalloys, a dispersion of a thorium bismuthide compound is formed, thethoriumlead compounds forming only in bismuth-leadalloy containing morethan about percent by weight of lead, the balance bismuth.

The dispersion process is carried out by heating one part by weight ofthe thorium sponge together with from 5 to parts by Weight of pieces ofbismuth, lead or bismuth-lead alloy. The metals are placed in a suitablecontainer such as a graphite crucible and heated in a furnace providedwith means of maintaining an inert atmosphere. At a temperature in therange of 550 to 650 C. the thorium forms thorium bismuthide orthorium-lead compound which spalls off the surface of the sponge untilthe conversion to intermetallic compound is completed. At temperaturesabove about 700 C. appreciable quantities of thorium tend to dissolve inthe hismuth, lead, or bismuth-lead alloy, and on cooling the resultingthorium low-melting metal alloy very large platelets of thorium compoundare precipitated. Dispersion of 10 to 20 gram quantities of thoriumsponge in 100 to 200 grams of bismuth, lead, or bismuth-lead alloy isgenerally completed in about 45 minutes to one hour at 600 C. Largerquantities of sponge and low-melting metal require longer times.

The so-prepared dispersion of thorium intermetallic compound may be useddirectly as a blanket in a breedertype nuclear reactor. Or if desired,it may first be further diluted with additional bismuth, lead, orbismuth-lead to accommodate a given reactor design.

Example As an example of the process of the invention amagnesium-thorium alloy was prepared by melting together thorium andmagnesium under an inert atmosphere and decanting the alloy from thedross. Forty and five-tenths (40.5) grams of the resulting alloyconsisting of about 30 percent by weight of thorium, the balancemagnesium, was placed in a molybdenum boat-type sublimer and thesublimer furnace was evacuated to about 0.020 mm. Hg pressure. Thefurnace was first brought to a temperature of 550 C. and then thetemperature was brought, over the space of five hours, to 700 C. The 700C. temperature was maintained for about seven hours to assure thecomplete removal of the magnesium as well as the formation of a coherentsponge weighing about 13.5 grams. The thorium sponge was placed in agraphite crucible along with .125 gram of oxide-free, 0.5 inch diameterbismuth rods. The crucible and contents were placed in an evacuableelectric furnace. The furnace was purged of air and filled with an inertgas. Then the furnace was brought to and maintained at a temperature of600 C. for one hour. During the one hour period the bismuththoriummixture was agitated several times. The furnace was then allowed to cooland the contents of the crucible solidified as a casting. The castingwas removed from the crucible, sectioned, and examined by themetallographic method. The casting was found to consist of a finelydivided dispersion in bismuth of thorium bismuthide particles whichexhibited a maximum dimension of about 10 microns. Chemical examinationof the casting indicated the concentration of residual magnesium wasless than 200 parts per million.

We claim:

1. In the method of preparing a fine particulate dispersion of a thoriumintermetallic compound selected from the group consisting ofthorium-bismuth and thorium-lead intermetallic compounds, in alow-melting metal selected from the group consisting of bismuth, leadand bismuth-lead alloys, wherein a thorium sponge is prepared and heatedin contact with said low-melting metal, the improved method of preparingsaid thorium sponge which comprises: alloying a magnesium metal selectedfrom the group consisting of magnesium, and magnesium containing up toweight percent of calcium, sodium, zinc and mixtures thereof, withthorium under an inert atmosphere, the amount of magnesium metal in thesoprepared thorium alloy being about 50 to 90 percent by weight of thealloy; and under a pressure of less than about 0.1 mm. of Hg, heatingsaid alloy initially to a temperature of about 550 C. and thereaftergradually to a temperature in the range of 650 to 900 (3., thereby tovolatilize said magnesium metal from the alloy and leave a thoriumsponge.

2. In the method of preparing a fine particulate dispersion of a thoriumintermetallic compound selected from the group consisting ofthorium-bismuth and tho rium-lead intermetallic compounds, in alow-melting metal selected from the group consisting of bismuth, leadand bismuth-lead alloys, wherein a thorium sponge is prepared and heatedin contact with said low melting metal, the improved method of preparingsaid thorium sponge which comprises: providing an alloy consisting ofthorium and a magnesium metal selected from the group consisting ofmagnesium, and magnesium alloyed with up to 5 weight percent of calcium,sodium, zinc, and mixtures thereof, the proportion of said magnesiummetal in the alloy being about 50 to 90 percent by weight of the alloy,the balance thorium; and under a pressure of less than about 0.1 mm. ofHg, heating said alloy initially to a temperature of about 550 C. andgradually to a temperature in the range of 650 to 900 C., thereby tovolatilize the magnesium metal from the alloy and leave a thoriumsponge. I

3. The method of preparing a fine particulate dispersion of a thoriumintermetallic compound selected from the group consisting ofthorium-bismuth and thoriumlead intermetallic compounds, in alow-melting metal selected from the group consisting of bismuth, leadand bismuth-lead alloys, which comprises: alloying a magnesium metalselected from the group consisting of magnesium, and magnesium alloyedwithup to 5 weight percent of calcium, sodium, zinc, and mixturesthereof, with thorium under an inert atmosphere, the amount of saidmagnesium metal in the so-prepared alloy being about 50 to percent byweight of the alloy, the balance thorium; heating said alloy under apressure of less than about 0.1 mm. of Hg, initially to a temperature ofabout 550 C. and gradually to a temperature in the range of 650 to 900C., thereby to volatilize said magnesium metal from the alloy and leave-a thorium sponge having an average cellular wall thickness of less thanabout 0.05 inch; contacting said thorium sponge with from 5 to parts byweight of said low-melting metal per part of thorium at a temperatureabove the melting point of the low-melting metal and below about 700 C.for a time sufficient for the thorium sponge to form an intermetalliccompound of thorium and said low melting metal, thereby to precipitatesaid intermetallic compound in the form of solid particles having amaximum dimension of about 1 millimeter.

4. The method of preparing a fine particulate dispersion of thoriumbismuthide in a low-melting metal selected from the group consisting ofbismuth, and bismuthlead alloy containing up to 90 weight percent lead,the balance bismuth, which comprises: contacting a thorium sponge-likemass having an average cellular wall thickness of less than about 0.05inch withfrom 5 to 100 parts by weight of said low-melting metal perpart of thorium sponge-like mass under an inert atmosphere, at atemperature above the melting point of the low-meling metal and belowabout 700 C. for a time suflicient for said thorium sponge-like mass toform the intermetallic compound thorium bismuthide, thereby toprecipitate said intermetallic compound in the low melting metal in theform of solid particles having a maximum dimension of about 1millimeter.

References Cited in the file of this patent UNITED STATES PATENTS2,434,775 Sosnick J an. 20, 1948 2,910,417 Teitel Oct. 27, 19592,915,445 Bryner Dec. 1, 1959 2,952,508 Bryner et al. Sept. 20, 1960

1. IN THE METHOD OF PREPARING A FINE PARTICULATE DISPERSION OF A THORIUMINTERMETALLIC COMPOUND SELECTED FROM THE GROUP CONSISTING OFTHORIUM-BISMUTH AND THORIUM-LEAD INTERMETALLIC COMPOUNDS, IN ALOW-MELTING METAL SELECTED FROM THE GROUP CONSISTING OF BISMUTH, LEADAND BISMUTH-LEAD ALLOYS, WHEREIN A THORIUM SPONGE IS PREPARED AND HEATEDIN CONTACT WITH SAID LOW-MELTING METAL, THE IMPROVED METHOD OF PREPARINGSAID THORIUM SPONGE WHICH COMPRISES: ALLOYING A MAGNESIUM METAL SELECTEDFROM THE GROUP CONSISTING OF MAGNESIUM, AND MAGNESIUM CONTAINING UP TO 5WEIGHT PERCENT OF CALCIUM, SODIUM, ZINC AND MIXTURES THEREOF, WITHTHORIUM UNDER AN INERT ATMOSPHERE, THE AMOUNT OF MAGNESIUM METAL IN THESOPREPARED THORIUM ALLOY BEING ABOUT 50 TO 90 PERCENT BY WEIGHT OF THEALLOY; AND UNDER A PRESSURE OF LESS THAN ABOUT 0.1 MM. OF HG, HEATINGSAID ALLOY INITIALLY TO A TEMPERATURE OF ABOUT 550*C. AND THEREAFTERGRADUALLY TO A TEMPERATURE IN THE RANGE OF 650 TO 900*C., THEREBY TOVOLATILIZE SAID MAGNESIUM METAL FROM THE ALLOY AND LEAVE A THORIUMSPONGE.